The present disclosure relates to a rotation detection mechanism that detects rotation of a rotary body to be heated, such as a fixing roller or the like, and is used in a fixing device that is mounted in an image forming apparatus such as a copy machine, a printer, or the like and fixes an unfixed image.
In an electrophotographic image forming apparatus, a roller heating system has been widely used in which at least one roller (a fixing roller) of a roller pair forming a nip is heated, and a recording medium (a paper sheet) carrying an unfixed toner image is passed through the nip of the roller pair so that toner is fixed on the recording medium. Furthermore, a belt heating system has also been used in which, in place of a fixing roller, an endless fixing belt is used and caused to generate heat by a heating unit, and at a nip between the fixing belt and a pressing roller, an unfixed toner image is fixed on a recording medium via the fixing belt.
As a method for heating such a fixing roller, a fixing belt, or the like, there are known a lamp method in which heating is performed by using a lamp, such as a halogen lamp or the like, which is disposed on an inner side of the roller or the belt and an induction heating (IH) method in which, in response to requests for a reduction in warm-up time and energy saving, heating is performed by using an eddy current generated by interlinking an alternating magnetic field with a magnetic conductor.
By the way, in a case of heating the fixing roller (or the fixing belt), generally, in order to prevent temperature unevenness in a circumferential direction of the roller (or the belt) from occurring, the roller (or the belt) is heated while rotating.
Particularly in the IH method, an induction heating portion is provided with an excitation coil and a core and thus is increased in size, because of which, in many cases, the induction heating portion is disposed outside the fixing roller or the fixing belt, so that the roller or the belt is heated from outside. In this case, when the roller or the belt starts to be heated while not rotating, disadvantageously, only part of a surface of the roller or the belt is heated. Furthermore, even though a temperature detection sensor that detects a surface temperature of the roller or the belt is disposed, in a case where the induction heating portion and the temperature detection sensor are disposed at different positions from each other in the circumferential direction, an abnormal temperature rise cannot be detected, so that there is a fear that thermal deformation of the roller or the belt, smoke generation, or ignition might occur. For this reason, it is required that the induction heating portion be energized while the roller or the belt is being rotated.
For example, there is known a fixing device that, by using a rotation detection unit that is provided with an optical sensor (a photointerrupter sensor) and a pulse plate, detects whether or not a heating roller is rotating. In a method described above, in conjunction with driving a fixing roller or the heating roller to rotate, the pulse plate rotates to open or interrupt an optical path of a detection portion of the optical sensor, based on which whether or not the heating roller is rotating is detected.